The present invention relates to the technique of phosphating metallic surfaces in nonaqueous phosphating baths.
It is known from experience with the conventional phosphating process (see W. Rausch, "Die Phosphatierung von Metallen" [The Phosphating of Metals], Eugen G. Leuze Publishers, Saulgau (1974), page 103), using conventional aqueous phosphating baths based on the ammonium or alkali dihydrogen phosphates--the so-called Fe phosphating process--that phosphate layer thicknesses of 0.3 .mu.m up to about 0.8 .mu.m are attainable by dipping methods, depending upon the choice of a dipping time in the range of about 2-5 minutes. An extension of the dipping period past this time does not result in a thickening of the phosphate layer. Only single-dip processes are known.
Also in the conventional, phosphating method (see W. Rausch, "Die Phosphatierung von Metallan", Eugen G. Leuze Publishers, Saulgau (1974), page 42) based on aqueous zinc phosphate, zinc iron phosphate, or zinc calcium phosphate solutions--the so-called Zn phosphating process--only the one-time dipping of the article to be phosphated has been used. In this process, phosphate layer thicknesses of about 1 .mu.m to about 20 .mu.m are produced, depending on the usage application, using dipping periods of 5-10 minutes or by spraying methods; layer thicknesses of about 2 .mu.m to 3 .mu.m are preferred.
Also, for the phosphating methods based on organic solvents--the so-called solvent phosphating process--especially those based on low-boiling halogenated hydrocarbons, which have become increasingly popular in recent years, only one-time dipping processes have been disclosed. In this connection, the dipping period is normally 0.5-3 minutes, reaching, in general, layer thicknesses of 0.1 .mu.m to about 1 .mu.m, depending on the dipping time and the composition of the organic phosphating bath. In individual cases, larger layer thicknesses are also attainable.
In order to evaluate the quality of phosphate layers on metallic surfaces as corrosion protection and/or as inorganic primer coatings for subsequent varnishing, the layer thickness alone is not an adequate criterion; rather, decisive factors also include porosity, surface roughness, crystallinity, water solubility, adhesive strength with respect to the metal surface, adhesiveness to the varnish coat, and other surface-specific properties. Only the combined effects of all surface and layer properties can determine the corrosion protection and the suitability as primer coatings.
For the evaluation of phosphate layers, empirical testing methods are generally employed after a distinct varnish coating step, such as, for example, the salt spray mist test on scratched test panels according to DIN [German Industrial Standard] 50 021 and DIN 53 167; the criss-cross cutting test according to DIN 53 151; the determination of the extent of rusting according to DIN 53 210; the determination of the degree of blistering (pimpling) according to DIN 53 209; and other test methods based on a given application.
The use of such testing methods for conventionally Fe-phosphated surfaces shows that the aqueous Fe-phosphating offers only a minor corrosion protection. In many cases, the requirements to be met by utilitarian articles and/or technical components are not satisfied.
In such cases, the conventional Zn-phosphating method is presently customarily employed, yielding a significantly better corrosion protection. However, Zn-phosphating, as compared with Fe-phosphating, is considerably less economical and represents a greater threat to the environment due to increased sludge formation.
In the more recent phosphating methods based on organic solvents, especially those based on low-boiling halogenated hydrocarbons, such as described, for example, in DAS 2,611,789, DAS 2,611,790, or European Patent Application 34,842, phosphating reactions similar to those of aqueous Fe-phosphating reactions are involved. Therefore the quality of the phosphate layers corresponds essentially to the quality of the conventional Fe-phosphating process. In many instances, therefore, the phosphate layers from the solvent phosphating process, just as the phosphate layers obtained by the conventional Fe-phosphating method, do not meet the posed requirements.
As is known, the solvent phosphating procedure, however, offers considerable advantages as compared with the conventional aqueous phosphating processes. For example, there is no environmental pollution by wastewater; the number of treatment steps is lower due to the elimination of various washing and rinsing steps; and the furnace drying step, which requires a large amount of energy, is unnecessary.